The present invention relates to a processed whey protein and a process for manufacturing the processed whey protein. The processed whey protein of the present invention is useful as a food material for manufacturing foods requiring such properties as gelling capability, water retention capacity, and high viscosity.
Whey proteins have conventionally been used as foodstuffs because of their high nutritional value and have also been widely used as supplemental materials for foods such as binders, extenders, and water retention agents, wherein the properties of the whey protein such as emulsifying property, foaming property, and gelation are utilized. Among these, the high gelation makes the whey protein an advantageous texture modifier for raw meat and fish meat products. Thus, the gelation of whey proteins is one important factor that improves the texture and water retention capacity of foods. Generally, proteins are denatured by heat and create mutual interactions such as a hydrophobic interaction between the protein molecules"" non-covalent bonds, such as ionic bonds and hydrogen bonds and an SH/SS exchange reaction whereas whey proteins are denatured and gelated by heat at temperatures of 60xc2x0 C. or higher. However, because the whey protein gel thus obtained is generally non-transparent and has only a small water retention capacity and a brittle structure, such a whey protein gel is not preferably used as a food material.
For these reasons, in order to improve the gel structure of whey proteins, a partially heat-denatured whey protein solution produced by partially denaturing the whey protein by heating, or a solution obtained by drying this partially heat-denatured whey protein solution to produce a dried powder and re-dissolving the dried powder, is used to obtain a whey protein gel which has high water-retention capacity and excellent texture. Specifically, a highly elastic gel with a high water retention capacity can be obtained by producing a solution of the partially heat-denatured whey protein at a concentration of 4-15% by weight, preferably 5-12% by weight and heating this solution at 55xc2x0 C. to 120xc2x0 C., preferably 65xc2x0 C. to 95xc2x0 C., or producing a solution by drying this solution and re-dissolving the resulting dried powder, and then by adding a salt at low temperatures to these solutions (Japanese Published Unexamined Patent Application No. 64550/1993), by acidifying these solutions (Japanese Published Unexamined Patent Application No. 124067/1990), or by freeze-drying and thawing these solutions (Japanese Published Unexamined Patent Applications No. 280834/1991 and 27249/1991). Whey proteins which are usually spherical produce a soluble aggregate in which protein molecules are aggregated like chains, if partially denatured by heating. This soluble aggregate in which protein molecules of the whey protein are aggregated like chains are hereinafter referred to as xe2x80x9csoluble aggregatexe2x80x9d. Although the whey protein does not gel in the soluble aggregate, this soluble aggregate of whey protein forms a three dimensional network and produces irreversible gel if the above-mentioned measures are taken. The whey protein gel thus obtained has a high water-retention capacity, superior elasticity, and a smooth constitution.
This soluble aggregate of whey protein can also be produced by preparing a whey protein solution with a concentration at which gelling does not occur by heating, and treating this solution with heat at 55xc2x0 C. to 120xc2x0 C. and for 1 to 120 minutes. This soluble aggregate of whey protein is referred to as xe2x80x9cpartially heat-denatured whey proteinxe2x80x9d in the present invention.
The gel produced from the solution of the partially heat-denatured whey protein which has been obtained by partially denatured whey protein with heat has a good constitution. However, a solution of the partially heat-denatured whey protein tends to change its form into a sol-like material due to an increase in its viscosity during storage for a long period of time, because the whey protein which has been partially denatured and is present as a soluble aggregate readily reacts with a small amount of a salt in the solution.
The inventors of the present invention have undertaken extensive studies to improve the shelf life of the solution of the above-mentioned partially heat-denatured whey protein. As a result, the inventors have found that if a casein protein is added to a solution of a partially heat-denatured whey protein wherein whey proteins are in the form of a soluble aggregate, the shelf life of the partially heat-denatured whey protein solution is improved, and a gel obtained from this whey protein exhibits a high water-retention capacity, excellent elasticity, and smooth constitution even if the solution of partially heat-denatured whey protein is stored for a long period of time. These findings have led to the completion of the present invention. Accordingly, an object of the present invention is to provide a processed whey protein with improved shelf life, which is capable of producing a gel exhibiting a high water-retention capacity, excellent elasticity, and smooth constitution, and to provide a process for manufacturing such a processed whey protein.